A known type of vehicle door assembly is disclosed in Japanese Utility Model Laid-Open Publication No. 62(1987)-189921. This vehicle door assembly includes a door inner panel and a base panel. The door inner panel possesses a frame shape and the base panel possesses an X-shape having four arm portions. The four arms of the base panel are fixed by welding to the four corner portions of the door inner panel so that the base panel is united to the door inner panel by the four arms. Therefore, the strength of the vehicle door assembly is secured by the door inner panel and the base panel.
In the above-described vehicle door assembly, a pair of door hinge devices, a door lock device and the door outer panel are adapted to be mounted on the door inner panel. By virtue of this construction, the attachment strength of the door hinge devices and the lock device is secured by only the door inner panel. Because the four arms of the base panel are fixed to the four corner portions of the door inner panel, the thickness of the inner panel must be increased to provide sufficient strength. Therefore, the thickness of the door inner panel is increased. As a result, the weight of the vehicle door assembly is increased.
In light of the foregoing, a need exists for a vehicle door assembly having sufficient strength, but without a significant increase in weight.